Apparatus for grinding mineral materials

ABSTRACT

Granular mineral materials are ground in a rotating mill having at least one partially confined grinding compartment. The material is fed through a constricted opening into one end of this compartment, atmospheric air being permitted to enter in the same way. The coarse material is gradually ground down into finely divided form and is discharged through a constricted opening into an adjoining collection compartment and thence discharged from the mill. The rate of feed of the coarse material into the grinding compartment and the rate of discharge of the ground material therefrom are so controlled that a pool of the finely ground material is maintained in the grinding compartment which is so fluidized that it behaves like a liquid. The apparatus is a tube mill having one or more grinding compartments. If there is only one grinding compartment a narrow collection compartment is located on the downstream side and if there are two grinding compartments the second one is on the downstream side of the narrow compartment. The wall of this compartment adjacent the grinding compartment includes a screen section, and the opposite wall a dam ring and overflow opening.

United States Patent [72] Inventor Gunner Rindal FagerholtCopenhagen-Valby, Denmark [2l Appl. No. 790,020 [22] Filed Jan. 9, 1969[45] Patented Jan. 11, 1972 [73] Assignee F. L. Smldth & Co.

New York, NY. [32] Priority Jan. 16, 1968 33] Great Britain [31 2,366/68[54] APPARATUS FOR GRINDING MINERAL MATERIALS 4 Claims, 8 Drawing Figs.

[52] U.S. Cl 241/70, 241/176 [51] Int. Cl B02c 17/06 [50] Field ofSearch 241/45, 49, 54, 70, 71, 72, 98,153, 171,176-178 [56] ReferencesCited UNITED STATES PATENTS 1,636,138 7/1927 Koppen..' 241/72 1,739,85512/1929 Newhouse 241/72X 2,185,96O 1 /1940 V ogel-Jorgensen 241 /54 x3,144,2l2 8/l964 Klovers 24l/72X Primary Examiner- Donald (1. KellyAttorney-Pennie, Edmonds, Morton, Taylor and Adams ABSTRACT: Granularmineral materials are ground in a rotating mill having at least onepartially confined grinding compartment, The material is fed through aconstricted opening into one end of this compartment, atmospheric airbeing permitted to enter in the same way. The coarse material isgradually ground down into finely divided form and is discharged througha constricted opening into an adjoining collection compartment andthence discharged from the mill. The rate of feed of the coarse materialinto the grinding compartment and the rate of discharge of the groundmaterial therefrom are so controlled that a pool of the finely groundmaterial is maintained in the grinding compartment which is so fluidizedthat it behaves like a liquid. The apparatus is a tube mill having oneor more grinding compartments. lf there is only one grinding compartmenta narrow collection compartment is located on the downstream side and ifthere are two grinding compartments the second one is on the downstreamside of the narrow compartment. The wall of this compartment adjacentthe grinding compartment includes a screen section, and the oppositewall a dam ring and overflow opening.

PATENTEU JAN] I {972 SHEET 1 OF 2 FIG.E

INVENTOR V GUNNAR R. FAGERHOLT ATTORNEYS PATENTED JAN] 1 4922 3.633832SHEET 2 OF 2 INVENTOR GUNNAR R.FAGERHOLT AT ORNEYS APPARATUS FORGRINDING MINERAL MATERIALS BACKGROUND OF THE INVENTION The dry grindingof coarse granular materials, such for instance as cement clinker, iscommonly efiected in tube mills containing grinding bodies. Such a millmay have only one grinding compartment, but more often it has two ormore grinding compartments each separated from the next by a diaphragmthrough which sufficiently ground material can pass. It has hithertobeen considered important to insure that material sufficiently ground topass through the diaphragm provided between the first compartment andthe next is removed as fast as it is produced by the impact of thegrinding bodies. Otherwise thefine ground material is thought tointerfere with the grinding by building up a protective layer on thegrinding-bodies and the coarse material.

When the first grinding compartment is overfilled, caused either by anincreased feed of coarse material or by a decrease in the discharge ofground material from the mill, backspill may occur. On the other hand,if the feed is too slow, less finished product is produced, and the wearon the grinding bodies and mill lining, which is always considerable, isincreased. There is an optimum rate of feed which should be maintained.

The mill produces a great deal of noise in operation, so much in fact asfrequently to be a nuisance to the mill operators and attendants and mayeven be dangerous to their health. In some cases the noise problem maybe solved by insulating the mill proper, for example by interposinginsulating material between the mill shell and the lining, or by makinga separate building enclosing the mill and isolating it from thesurroundings. The noise serves a useful purpose, however, in that itincreases if the feed is too slow and decreases if it is too fast. Anexperienced mill operator is capable of controlling the feed to the millaccording to the grinding noise emitted from the mill. Moreover, thecontrol can be made automatic by equipping the grinding plant with amicrophone, a transmitter and an amplifier by means of which the soundis converted into a suitable control signal which alone or incombination with other signals is used to control the feed to the mill.

SUMMARY OF THE INVENTION According to the invention the rate of feed tothe mill through a constricted opening in one end wall of the mill andthe rate of discharge from the first or sole grinding compartment are socontrolled that part of the ground product, although ground to such anextent as to be capable of passing out of the grinding compartment, isprevented from leaving it, with the result that a permanent pool ofground material forms and is maintained at the bottom portion of theconfined grinding compartment. This pool is so aerated and fluidizedduring the motion of the charge by air already in and entering thegrinding compartment as to behave as a liquid. The finely groundmaterial from the pool is gradually screened out at the discharge end ofthe grinding compartment and collected in the adjoining narrowcompartment. The material is gradually removed from the collectioncompartment through a constricted discharge opening. This rate ofdischarge controls the rate of feed of coarse material to the confinedgrinding compartment.

We find that the wear is reduced because of the high proportion of finematerial that is present in the grinding compartment, and that there isan increase in the efiiciency as measured by the power consumption. Inaddition the noise is reduced, and the grinding process can be carriedon with a grinding noise lower than was previously considered desirable,and yet the feed to the mill can still be controlled by the noise.

In order to maintain the pool of ground material in the first or solegrinding compartment, it is necessary to construct the millappropriately. The invention includes novel tube mill constructionssuitable for carrying out the method and including a relatively narrowcompartment located downstream of the first or sole grinding compartmentand adapted to serve as a reservoir of fluidized ground material. Thepartition between the first or sole grinding compartment and the narrowreservoir compartment is constructed to enable the fluidized groundmaterial to flow from either compartment to the other. The narrowreservoir compartment is bounded downstream by a wall having one or moreopenings through which the ground material can pass and which are solocated that the continued grinding operation produces a permanent poolof substantial depth which is maintained in the narrow reservoircompartment.

If there is only one grinding compartment, the wall bounding the narrowreservoir compartment downstream is constituted by the end wall of themill containing the discharge opening which is through a hollowsupporting trunnion for the mill tube. More commonly, however, tubemills have more than one grinding zone or compartment, and in this casethe narrow reservoir compartment is preferably intermediate the firstand second grinding compartments and is bounded downstream as well asupstream by a partition.

The provision of an intermediate compartment in a mill is known, buthitherto the reason for providing it has been to improve the movement ofthe ground product through the mill, and lifters or the equivalent havebeen provided for discharging the intermediate compartment as fully andquickly as possible. In the present invention there are no such lifters,but there may advantageously be scoops working in a central area tolimit the depth of the reservoir of fluidized material.

It is found in practice that there is sometimes a tendency for the poolin the narrow compartment to be undesirably reduced as a result ofdischarge of material at a greater rate than that of entry, and indeedthe narrow compartment is sometimes emptied. The reason is that somematerial is positively lifted above the opening in the downstream wallor partition wall and on falling down again passes through the opening.The wall or partition is of course built up from individual elements,and bolt heads or other projections tend to lift the material,particularly when this is sticky, for example lime.

Such undesired discharge from the narrow compartment may be prevented byshrouding the central opening through which the discharge takes place sothat material falling downwards is deflected and so prevented frompassing through the opening. The tendency to the undesired discharge maybe reduced also by increasing the fluidization of the pool in theintermediate compartment. This may be done by providing stirrers or byintroducing fluidizing air through airpermeable plates at the peripheralwall of the intermediate compartment.

BRIEF DESCRIPTION OF THE DRAWINGS Several embodiments of the inventionare shown by way of example in the accompanying diagrammatic drawings,in which FIG. 1 is a vertical section through the inlet end and thefirst grinding compartment of one multicompartment mill;

FIG. 2 shows a vertical section through the inlet end and the firstgrinding compartment of another mill;

FIG. 3 shows a modification of the mill of FIG. 1;

FIG. 4 is a section on the line 4-4 of FIG. 3;

FIG. 5 is a section on the line 55 of FIG. 3;

FIG. 6 shows the distribution of the charge and material in the grindingcompartment diagrammatically;

FIG. 7 is a vertical section similar to FIG. 1 through a modified mill;and

FIG. 8 is a section on the line 8-8 ofFIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiment shown in FIG. 1includes a mill shell 1 or tube 1 with a wear-resistant lining 2 and iscarried by hollow trunnions, of which only the trunnion 3 at the inletend is shown. These hollow trunnions form constricted inlet anddischarge openings for the material. Material to be treated in the millis fed through the central opening 4 of one of the trunnions into afirst grinding compartment 5, which is separated from a second grindingcompartment 7 by a narrow intermediate reservoir compartment 6 boundedby two partitions 8 and 9. The partition 8 comprises an imperforate rimportion 16 and a center portion 17 with an annular screen portion 15between them. The partition 9 is a dam ring with a single centralopening 14. The meshes of the screen 15 allow ground material but notgrinding bodies to pass.

Coarse material to be ground is fed substantially continuously throughthe opening 4 to the first grinding zone or compartment '5, whichcontains a suitable amount of grinding bodies (not shown). The groundmaterial can leave the grinding compartment through the screen inpartition 8, and

pass into the intermediate reservoir compartment6. As it can leave thisonly by overflow through the central opening 14 of dam ring 9, areservoir of ground material is maintained in the intermediatecompartment 6. The charge in the grinding zone or compartment 5 iscomposed of two parts as indicated in FIG. 6. One part 19 comprises amixture of grinding bodies and of material being treated in variousstages of reduction. Because of the rotation of the mill, indicated bythe arrow, and the friction between the lining 2 and the charge, thispart 19 is in continuous motion, being lifted above the axis of the milland then sliding and tumbling downwards.

The second part of the charge in compartment 5 consists almostexclusively of ground product which is fluidized by air during theviolent agitation of the charge. The fluidization is assisted byventilating air which is drawn through the mill as usual. The fluidizedmass of ground product forms a pool 20 which behaves like a liquid, andif it could pass freely out of the grinding compartment 5 it would doso. However the reservoir of material in intermediate compartment 6inhibits the passage of the ground material into the intermediatecompartment, except at the same rate as ground material passes throughthe opening 14 in the dam ring 9, and causes ground material to build upto the same level against the partition 8 in the grinding compartment 5.Thus, contrary to conventional practice, the first grinding compartment5 is permanently overfllled.

The existence of the pool 20 of fluidized ground material in grindingcompartment 5 is advantageous as it protects the wearing plates at thetoe of the mixture 19 of grinding bodies and coarse material against theimpact of the grinding bodies. Part of the ground material iscontinually lifted or pumped out of pool 20 so as to form a constituentof the part 19 and partake in the continuous movement of this part.Ground material continuously leaves this part 19 to enter theintermediate reservoir compartment 6. In addition the pool reduces thenoise.

In FIG. 2 two different partitions 10 and 11 bounding the intermediatereservoir compartment 6 are shown. The partition 10 includes a screen 21that extends as close as is feasible to the mill lining 2, and thepartition 11 is a dam ring with a screen 22 over its central opening.This screen 22 serves to prevent grinding bodies in the second grindingcompartment 7 from entering the intermediate reservoir compartment 6.

If it is desired to increase the ease of flow of ventilating air throughthe mill, it is desirable to use a partition such as 10 with itsincreased area of opening, rather than that shown at 8, and to use apartition such as dam ring 9 which has no screen.

Increased control over the level of the fluidized material in theintermediate compartment 6, and therefore over the volume of the pool 20of fluidized material, can be obtained by the provision of apparatus fordischarging material from a central zone in the intermediate compartment6. One such apparatus is shown at 12 in FIGS. 3 and 5 and comprisesscoops 13 carried by radial vanes 18 which are mounted on supports notshown and which guide the scooped material to the opening 14 in thepartition or dam ring 9. These scoops 13 cannot empty the intermediatecompartment 6 of ground material, but in practice they maintain it at adesired level. Their outer ends should be located at a substantialdistance from the wall of the mill, and the desired level can be adusted by cutting off part of the end of each scoop after the apparatushas been installed in the mill.

FIGS. 7 and 8 diagrammatically show a construction by which the tendencyfor the pool of fluidized material in the intermediate compartment 6 tobe undesirably reduced or even emptied is effectively eliminated. Inthis construction there is an intermediate compartment 23 bounded bypartitions 24 and 25 which are joined by staybolts 26. The partition 24has a solid center portion 27 and rim portion 28 with an annular screen29 between them. The center 27 carries a frustoconical ring 30. Thepartition 25 has a central opening 31, which is surrounded within thecompartment 23 by a frustoconical ring 32 which embraces the ring 30.

FIG. 8 shows a fluidized pool 33 from which material tends to be carriedupwards as shown at 34 to fall downwards as shown at 35. It will be seenthat the rings 32 and 30 effectively shroud the opening 31, whileallowing material from the surface of the pool to pass. The staybolts 26stir the pool and help to keep it fluidized.

I claim:

1. A tube mill having at least one grinding compartment with grindingbodies therein, means for maintaining a pool of fluidized groundmaterial therein including a relatively narrow collection compartmentlocated downstream from the grinding compartment and serving as areservoir of fluidized ground material, and a screen between thegrinding compartment and the narrow collection compartment constructedto allow the fluidized ground material, but not grinding bodies, to passfrom either compartment to the other, and the narrow collectioncompartment being bounded downstream by a wall having at least oneopening through which the ground material can pass to maintain the depthof said pool desired.

2. A tube mill having at least one grinding compartment having grindingbodies therein and a permanent pool of fluidized ground material, arelatively narrow collection compartment located downstream from thegrinding compartment and serving as a reservoir of fluidized groundmaterial, a screen between the grinding compartment and the narrowcollection compartment constructed to allow the fluidized groundmaterial, but not grinding bodies, to pass from either compartment tothe other, and the narrow collection compartment being boundeddownstream by a wall having at least one opening through which theground material can pass, the collection compartment being provided withscoops the outer ends of which are located at such a distance from theperipheral wall of the mill that in operation a pool of substantialdepth always exists in the narrow collection compartment therebycontrolling the depth of the pool of ground material in the grindingcompartment.

3. A tube mill according to claim 2 having two grinding compartments anda relatively narrow collection compartment disposed between the twogrinding compartments, and a partition between the collection and secondgrinding compartments having a single-central opening such that inoperation a pool of ground material of substantial depth always existsin the intermediate compartment.

4. A tube mill according to claim 3 in which a shroud extends aroundsaid single central opening to deflect downwardly flowing material andpreventing it from passing through the opening in the backward directionfrom said second grinding compartment to said collection compartment.

1. A tube mill having at least one grinding compartment with grindingbodies therein, means for maintaining a pool of fluidized groundmaterial therein including a relatively narrow collection compartmentlocated downstream from the grinding compartment and serving as areservoir of fluidized ground material, and a screen between thegrinding compartment and the narrow collection compartment constructedto allow the fluidized ground material, but not grinding bodies, to passfrom either compartment to the other, and the narrow collectioncompartment being bounded downstream by a wall having at least oneopening through which the ground material can pass to maintain the depthof said pool desired.
 2. A tube mill having at least one grindingcompartment having grinding bodies therein and a permanent pool offluidized ground material, a relatIvely narrow collection compartmentlocated downstream from the grinding compartment and serving as areservoir of fluidized ground material, a screen between the grindingcompartment and the narrow collection compartment constructed to allowthe fluidized ground material, but not grinding bodies, to pass fromeither compartment to the other, and the narrow collection compartmentbeing bounded downstream by a wall having at least one opening throughwhich the ground material can pass, the collection compartment beingprovided with scoops the outer ends of which are located at such adistance from the peripheral wall of the mill that in operation a poolof substantial depth always exists in the narrow collection compartmentthereby controlling the depth of the pool of ground material in thegrinding compartment.
 3. A tube mill according to claim 2 having twogrinding compartments and a relatively narrow collection compartmentdisposed between the two grinding compartments, and a partition betweenthe collection and second grinding compartments having a single-centralopening such that in operation a pool of ground material of substantialdepth always exists in the intermediate compartment.
 4. A tube millaccording to claim 3 in which a shroud extends around said singlecentral opening to deflect downwardly flowing material and preventing itfrom passing through the opening in the backward direction from saidsecond grinding compartment to said collection compartment.